District of Columbia insurance officials who have long worried about how many people would sign up for health coverage get a surprise bonus next month: a slew of Congress members and staffers.

Three weeks from now, about 15,000 Hill aides and their bosses will be eligible to sign up through the small-business or SHOP exchange offered by the D.C. Health Link. Not all will enroll — some are on a spouse’s plan or have Medicare. But those who do will most likely be disproportionately young and healthy, reflecting the Capitol Hill workforce — exactly the kind of customers the insurance pool wants and needs.

They’ll provide a steady stream of customers for the D.C. exchange, which faces unusual challenges. At just more than 600,000 people, the District is smaller than most states. That makes it harder to gather a pool of healthy people to balance out the costs of sicker enrollees and bring down premiums. Plus a large cross section of those D.C. residents won’t go into the exchanges at all: One in three is on Medicaid. Many others who work for the government — but not on the Hill — will stay on federal health plans.

The District had considered requiring all small businesses that buy insurance to do so through the SHOP exchange but ended up creating a two-year transition period.

Exchange board member Mohammad Akhter has said the insurance pool needs about 100,000 people to be “successful.”

Hill aides may be ideal for getting SHOP off to a good start — although officials say it’s still too early to know how many will sign up or what the demographics will end up looking like.

And unless the enrollees from the Hill are dramatically younger and healthier, there probably won’t be enough of them to significantly change premiums for 2015 (they’re already fixed for next year), said Dan Bailey, a Society of Actuaries fellow who has worked with other state exchanges. It will depend on their age and gender.

But he warned that it’s hard to say at this point. “It’s so damn complicated,” he said.

D.C. exchange officials recognize it as a boon, but they’re not prematurely counting any wins. “Clearly, it will be more than we would have gotten without it,” exchange spokesman Richard Sorian said.

In the first week of enrollment, 426 small businesses and 12,294 individuals created accounts on D.C.’s small group market and individual exchanges, which will be consolidated into one pool. Members of Congress and staff can enroll from Nov. 9 until Dec. 11.

Hill staffers have already been scouting out their options. Some younger aides say they prefer the online enrollment to the paper process required to enroll in the Federal Employees Health Benefits program.

“I think it’s a little less intimidating” than picking an FEHB plan, she said, adding that she just “closed my eyes and picked” when she started her job earlier this year.

Some veteran staffers are frustrated that they may have to change health providers. Abby Gunderson, a senior legislative assistant for Rep. Scott Rigell (R-Va.), has worked on the Hill for six years and has been on the same federal health plan for most of that time.

Ordinarily, the federal government would continue to directly sponsor plans for employees just like large businesses are required to do under Obamacare. But there’s a special provision in the law requiring members of Congress and all aides — except for those who work for leadership or committees — to enroll in exchange coverage.

Exactly how that unusual situation would work wasn’t spelled out in the law, so the Obama administration had to clarify that point. Last month, the administration released a rule saying eligible members and staff must buy on the D.C. exchange, even if they live in another state.

The federal government will continue paying for about 75 percent of their plans. To get the contribution, the members and aides must purchase so-called gold plans, which cost the most but also pay a larger share of health costs.

The exchange will have to be tweaked to allow the federal contributions, since it’s set up to handle plan contributions only from small employers with 50 or fewer workers. Exchange officials said the changes are minimal.